What's this? somedomain.co.uk/this.dll?isIISapp=false?

I have seen urls like this on some websites can anyone tell me what the dll thing is?

Is it an IIS application?
Is it a component?
Is it just a .dll stored in the website?
Is it quicker than ASP ?

Could someone build one in VB?

Thanks in advance

I think it is a dll stored in a cgi-bin, but I believe you cannot write these in VB I think they have to be in C or Perl or something.